For reliable operation in a desired period of use of a micro component, for example a micro-electromechanical component such as a micro sensor or micro actuator, the atmosphere in the component is intended to be adjusted and maintained in accordance with the function. In numerous micro components, penetrating gases are harmful when the function of the component is based on controlled vacuum conditions or the maintenance of a specific pressure. It may also be necessary to form in the component an atmosphere with a specific composition over the entire period of use. In particular, oxygen, hydrogen and water vapour are undesirable in hermetic housings. In addition to the resultant pressure increase, however, gases can also activate other damaging mechanisms.
It is generally known to adjust and maintain the atmosphere in a micro component by introducing functional materials, in particular getter materials. The quantity of getter material to be used in order to produce, adjust and maintain the atmosphere is dependent on the absorption capacity thereof. This in turn is highly dependent on the exposed surface as an effective surface and reaction surface with gas molecules. In principle, it is known to obtain the effective surface by means of surface structuring of the substrate, porous or finely structured functional material films or getter films or by means of columnar grain structures of a thin getter film. With an integrated getter, the internal pressure increase is slowed down, since all active air gases are absorbed, and only the penetrated inert gas portion of the air leads to a pressure increase. An ideal getter without any saturation effect by taking up active air gases limits the internal pressure increase in the long term to approximately 9.3 mbar (sum of all inert gas partial pressures in the atmosphere). Consequently, the internal pressure is 100 times lower than in housings without any getter.
Another possibility here in addition to the use of a getter for achieving a sufficiently long period of use for the micro component is, for example, to double the housing inner volume. Owing to a more deeply etched cavity, the pressure increase or the change of atmosphere in the housing is slowed down, for example, by a half. However, for reasons of mechanical component stability and component size, this geometric approach often can no longer be used or can be used only in a limited manner.
The necessary capacity of the functional material in a hermetic housing can be defined by the fact that the functional material retains its respective function to a sufficient degree over the entire service-life of the component. In the case of a getter, its getter capacity can be defined by the fact that, in the event of an assessed critical air leakage rate, no getter saturation occurs within the guaranteed service-life of the component. Generally, the air leakage rates which can currently be verified are at best in the range of 10−14 mbar·l/s. Known thin layer getters must be integrated in the housing over a very large surface-area in order to provide the necessary getter capacity. The trend towards increasingly small component geometries is counterproductive with respect to a sufficiently long function of the functional or getter material since there is, for example, no sufficiently planar surface for arrangement of the material, whereby a critical situation may occur with components which require a longer service-life.
The introduction of functional materials or getters in microtechnical components can be carried out in different manners. At the beginning of miniaturised vacuum housing at wafer level, porous shaped components of the corresponding material were introduced into housing cavities provided specially for that purpose. The lateral arrangement often used in this instance disadvantageously increases the size of the component so that the quantity of the functional or getter material available in the housing is also limited. As an alternative housing construction, metal carrier films with sintered getter layers were welded, in the housing cover, for example, or impressed immediately as a thick layer into the housing cover and sintered therewith. The vertical arrangement saves space, but any particles which occur disadvantageously fall directly onto the sensitive component structure. The getter activation in both cases is generally carried out after hermetic housing closure by means of tempering in an oven (R. Kullberg et al., Getter for microelectronic packages, Advanced Packaging, 12/2004, pages 30-33). After component capping has been introduced at wafer level, getters were placed as an NEG shaped component in cavities which are reserved for them at the side of the resonator and which are connected thereto by means of channels. The development of structurally precipitatable thin film getters directly in recesses of the cap wafer allowed vacuum housings at wafer level with cavity volumes in the region of a few nanoliters (High vacuum wafer bonding technology, AuSi eutectic wafer bonding with integrated getter thin film for long term stable high vacuum, W. Reinert, MST News, special edition concerning wafer bond technology, February 2005).
The integration of a getter material or other functional material into a vacuum wafer bonding process for producing micro components places high demands on the material itself. It must behave passively during storage and wafer handling, there must be no wafer distortion owing to layer stress, the precipitation temperature must not be too high (<300° C.), the structuring thereof must not limit the cap wafer process (the production of the passive cap) with respect to the selection of metal and depth of the cavities, it must adhesively bond in an excellent manner and emit no particles, the characteristic features thereof must not change in a negative manner owing to wafer cleaning, no inert gas must be discharged during the wafer bonding operation, the activation temperature thereof should not be above joining temperature and the activation process should not last too long. In addition a getter should not already become saturated owing to the gas emissions during the wafer bonding operation, and any bonded gas must not be released again at normal operating temperatures of the component.
Another problem which occurs when getters are used is described below. Oxygen and nitrogen as gases to be absorbed are chemically bonded by the getter and converted to corresponding oxides or nitrides on the surface thereof. Since the oxides and nitrides formed take up a larger volume than the converted getter material, the chemical reaction of the getter is also associated with the development of mechanical stress which under some circumstances can exceed a critical level and can bring about flaking or similar material defects in the getter structure. Although the formation of the oxides and nitrides can also lead to the reactivity of the getter being limited, these layers hinder the further reaction of the getter with the gases. If a critical layer thickness is exceeded, the further reaction of the getter is stopped, whereby the getter has initially reached its maximum capacity.